When a load is driven by an output of a power supply portion with varying output voltage such as a generator driven by an internal combustion engine, a power supply device is used having a voltage control function of controlling to maintain the output voltage to be supplied to the load at a set value. Such a power supply device is comprised of a voltage converting portion that converts an input voltage supplied from the power supply portion into an output voltage to be supplied to a load, an output voltage setting portion that generates an output voltage setting signal for providing a set value of a voltage value of the output voltage, and an output voltage control portion that controls the voltage converting portion so as to make zero a deviation between the voltage value and the set value of the output voltage.
For example, a known power supply device is shown in FIG. 10 that converts an output voltage of a magneto generator driven by an internal combustion engine into a DC voltage and supplies the DC voltage to a load. This power supply device is disclosed in Japanese Patent Application Laid-Open Publication No. 8-33228. In FIG. 10, a reference numeral 1 denotes a power supply portion constituted by a magnet type AC generator driven by the internal combustion engine, 2 denotes a voltage converting portion that converts an input voltage supplied from the power supply portion 1 into an output voltage to be supplied to a load 3, 4′ denotes an output voltage setting portion, and 5 denotes an output voltage control portion that controls the voltage converting portion 2 so as to maintain the output voltage to be supplied to the load 3 at a set value.
The voltage converting portion 2 is comprised of a circuit constituted by two rectifying diodes D1 and D2 and two MOSFETs F1 and F2 bridge-connected, and turns on/off the MOSFETs F1 and F2 to interrupt an armature current flowing through an armature coil 1A of the generator that constitutes the power supply portion 1 and increase an induced voltage of the armature coil. The increased induced voltage of the armature coil is rectified by a full-wave rectifier circuit constituted by the diodes D1 and D2 and the MOSFETs F1 and F2 and supplied to the load 3.
The output voltage setting portion 4′ is comprised of a voltage divider circuit 4A having resistances R1 and R2, and divides a constant DC voltage Vcc to generate an output voltage setting signal vs for providing a set value Vs of the output voltage.
The output voltage control portion 5 is comprised of a voltage detecting portion 5A, a drive signal generation circuit 5B, and a drive signal supply control portion 5C. The voltage detecting portion 5A is constituted by a voltage divider circuit having resistances R3 and R4, and detects an output voltage V1 obtained from the voltage converting portion 2 to generate a voltage detecting signal v1. The drive signal generation circuit 5B generates rectangular wave drive signals Vq provided to gates of the MOSFETs F1 and F2, and simultaneously provides the drive signals Vq to the MOSFETs F1 and F2 to simultaneously turn on/off the MOSFETs F1 and F2. The drive signal supply control portion 5C controls provision of the drive signals Vq from the drive signal generation circuit 5B to the MOSFETs F1 and F2 so as to make zero a deviation between the voltage value V1 of the output voltage detected by the voltage detecting signal v1 output from the voltage detecting portion 5A and the set value Vs of the output voltage provided by the output voltage setting signal vs.
The shown drive signal supply control portion 5C comprises a comparator CMP that compares the voltage detecting signal v1 with the setting signal vs, and controls the drive signal generation circuit 5B according to a magnitude relationship between the voltage detecting signal v1 and the setting signal vs.
The drive signal generation circuit 5B generates the rectangular wave signals Vq having a predetermined duty to simultaneously turn on/off the MOSFETs F1 and F2 at the predetermined duty when the voltage detecting signal v1 is lower than the output voltage setting signal vs and an output of the comparator CMP is at a high level, and sets the duty of the rectangular wave signals Vq to 100% (with the drive signals at a constant level being still provided to the gates of the MOSFETs F1 and F2) to maintain both the MOSFETs F1 and F2 in an ON state when the voltage detecting signal v1 becomes higher than the output voltage setting signal vs and the output of the comparator CMP is reduced to a low level.
When the MOSFETs F1 and F2 are simultaneously turned on, the armature coil 1A is short-circuited through the MOSFETs F1 and F2, and thus the armature current flows through the armature coil 1A. When the MOSFETs F1 and F2 are both turned off, the armature current having been flowing is interrupted, and thus an increased voltage is induced in the armature coil 1A. Thus, when the voltage detecting signal v1 is lower than the output voltage setting signal vs and the MOSFETs F1 and F2 are repeatedly turned on/off, the increased voltage is induced in the armature coil 1A every time the MOSFETs F1 and F2 are turned off, and the voltage is supplied to the load 3 through the full-wave rectifier circuit constituted by the diodes D1 and D2 and the MOSFETs F1 and F2 of the voltage converting portion 2.
When the output voltage V1 supplied from the voltage converting portion 2 to the load 3 exceeds the set value Vs, and the voltage detecting signal v1 exceeds the output voltage setting signal vs, the output of the comparator CMP is reduced to the low level. At this time, the drive signal generation circuit 5B has a duty of 100% of the drive signal provided to the MOSFETs F1 and F2. Thus, the armature coil 1A is maintained in a short-circuited state through the MOSFETs F1 and F2 to reduce the output voltage V1. Repeating these operations maintains the voltage value of the output voltage V1 to be supplied to the load 3 at around the set value.
As described above, the MOSFETs F1 and F2 are turned on/off to interrupt the armature current of the generator to induce the increased voltage in the armature coil, and rectify the increased voltage and supply the voltage to the load. This allows a high output voltage to be obtained during low speed rotation of the generator even with a reduced number of turns of the armature coil 1A. The reduced number of turns of the armature coil 1A can reduce impedance thereof, thereby allowing a high load current to be passed during high speed rotation of the generator.
In the power supply device as described above, variations in resistance values of the resistances R1 to R4 cause variations in values of the output voltage V1 supplied from the voltage converting portion 2 to the load 3. Thus, in order to keep the voltage value of the output voltage V1 of the power supply device within a specified range, adjustment of the resistance values of the resistors R1 to R4 or replacement of any of the resistors R1 to R4 is required when it is detected that the output voltage V1 falls outside the specified range in a test conducted in a test step in a production process of the power supply device, which requires further man-hours for adjustment of the output voltage and increases production costs.
High accuracy resistors with small variations in resistance value may be used as the resistors R1 to R4, but the high accuracy resistor is very expensive and use thereof increases costs of the power supply device.
An allowable range of variations in output voltage of the power supply device may be increased to some extent, but when the characteristic of the load 3 is changed according to the power supply voltage, control items of the load need to be corrected according to the value of the output voltage of the power supply device in a control device that controls the load 3, which is troublesome.
For example, when the load is an injector (a fuel injection valve that injects fuel to be supplied to the engine), ineffective injection time (delay time from when an injection command signal is provided to an injector drive circuit to when fuel injection is actually started) of the injector changes according to the power supply voltage. In a certain injector, ineffective injection time when a driving voltage is 14.0 V is 1000 μsec, while the ineffective injection time becomes 1150 μsec when the driving voltage decreases to 13.6 V. In this case, if signal widths of injection command signals provided to the injector are equalized, the ineffective injection time is increased when the power supply voltage is low, thereby reducing actual injection time and causing insufficient amount of fuel to be supplied to the engine. In order to prevent this, the ineffective injection time needs to be corrected according to the power supply voltage in the control device that controls the injector, which is troublesome. Particularly, when an inexpensive CPU without a dividing function is used in the control device, the correction cannot be properly performed in some cases.
The power supply device is shown in FIG. 10 as an example in which chopper control of the armature current is performed to induce the increased voltage in the armature coil, thereby obtaining the voltage for driving the load without increasing the number of turns of the armature coil of the generator. However, the same problem occurs in a power supply device of other type in which a set value of an output voltage is determined by a resistor, for example, a power supply device in which a voltage setting resistor is connected to a regulator IC to obtain a voltage adjusted to a predetermined set value, because variations in resistance values of the voltage setting resistor cause variations in output voltage.